EP3244281A1 - An on chip temperature independent current generator - Google Patents

Abstract

An on chip temperature independent current generator, TICG, (1) for generating a temperature independent current (I_out), said TICG (1) comprising an on chip current generator (3) having an output (4) to provide an electrical current (I_PTAT) having a current amplitude being proportional to a temperature (T) of said chip; an on chip transistor (9) having a base (B) connected to a temperature independent reference voltage generator (10), a collector (C) connected to a current mirror (12) and an emitter (E) connected to the output (4) of the on chip current generator (3) and connected via an on chip resistor (14) to a reference potential (GND); wherein the current mirror (12) is adapted to mirror a collector current (I_c) flowing to the collector (C) of said on chip transistor (9) to generate the temperature independent current (I_out).

Description

• The invention relates to an on chip temperature independent current generator for generating a temperature independent current which can be supplied to other circuit elements of an integrated circuit.
• Conventional current generators which can generate a temperature independent current can be based on voltage to current converter circuits and require a temperature independent reference voltage band gap as well as a temperature independent resistance. However, it is difficult to implement this kind of current generator in CMOS technology.
• Further, there are known conventional temperature independent current generators comprising current DACs and a set of current mirrors in which a first current proportional to the absolute temperature and a second current complementary to the absolute temperature are mixed in proper proportion to provide a temperature independent current. However, providing a temperature independent current this way requires a precise trimming of the temperature dependency compensation.
• Accordingly, it is an object to provide an on chip temperature independent current generator which does not require a trimming.
• This object is achieved by an on chip temperature independent current generator comprising the features of claim 1.
• The invention provides an on chip temperature independent current generator for generating a temperature independent current wherein said on chip temperature independent current generator comprises
  an on chip current generator having an output to provide an electrical current being proportional to an absolute temperature of the chip,
  an on chip transistor having a base connected to a temperature independent reference voltage generator, a collector connected to a current mirror and an emitter connected to the output of the on chip current generator and connected via an on chip resistor to a reference potential,
  wherein the current mirror is adapted to mirror a collector current flowing to the collector of said on chip transistor to generate the temperature independent current.
• In a possible embodiment of the on chip temperature independent current generator according to the present invention, the on chip transistor comprises an on chip bipolar NPN transistor.
• In a further possible embodiment of the on chip temperature independent current generator according to the present invention, the current mirror comprises a CMOS or BJT current mirror.
• In a further possible embodiment of the on chip temperature independent current generator according to the present invention, the on chip current generator comprises an operation amplifier having an inverting input to which a first bipolar transistor is connected and a non-inverting input to which a second bipolar transistor is connected via a resistor having a predetermined resistance and having an output connected to an integrated CMOS current mirror of said on chip current generator.
• In a further possible embodiment of the on chip temperature independent current generator according to the present invention, the on chip resistor is of the same type as the resistor of the on chip current generator.
• In a further possible embodiment of the on chip temperature independent current generator according to the present invention, the on chip resistor has a resistance being m times the resistance of the resistor of said on chip current generator, wherein m is a positive real number.
• In a still further possible embodiment of the on chip temperature independent current generator according to the first aspect of the present invention, the resistance of the resistor of said on chip current generator is dependent on the temperature of said chip.
• In a still further possible embodiment of the on chip temperature independent current generator according to the present invention, the resistance of the resistor of said on chip current generator is temperature independent.
• In a further possible embodiment of the on chip temperature independent current generator according to the present invention, the current generated by said temperature independent current generator is temperature independent in a wide temperature range between about -60° Celsius and about +200° Celsius.
• In a further possible embodiment of the on chip temperature independent current generator according to the present invention, the current generated by the temperature independent current generator comprises a nominal current amplitude in a range of about 0,6 to 1,0 µAmp.
• In the following, possible embodiments of the on chip temperature independent current generator according to the present invention are described in more detail with reference to the enclosed figures.

Fig. 1

shows a circuit diagram for illustrating a possible exemplary embodiment of an on chip temperature independent current generator according to the present invention;

Fig. 2

shows a circuit diagram of a possible exemplary implementation of an on chip current generator integrated in the on chip temperature independent current generator according to the present invention as illustrated in the embodiment of Fig. 1;

Fig. 3

shows a diagram for illustrating the operation of an on chip temperature independent current generator according to the present invention in comparison to a conventional current generator.

• As can be seen in Fig. 1, an on chip temperature independent current generator 1 is configured to generate a temperature independent current output by the on chip temperature independent current generator 1 at an output terminal 2 as illustrated in Fig. 1. The on chip temperature independent current generator 1 comprises in the illustrated embodiment an on chip current generator 3 having an output 4 to provide an electrical current I_PTAT having a current amplitude being proportional to an absolute temperature T of the chip. The output 4 of the on chip current generator 4 is connected via a line 5 to an internal node 6 connected to the emitter E of an on chip transistor 9 which is formed in the illustrated embodiment by an on chip bipolar NPN transistor. The on chip transistor 9 has a base B connected to a temperature independent reference voltage generator 10 via an internal line 11. The temperature independent reference voltage generator 10 can be in a possible embodiment formed by a band gap voltage generator. The on chip transistor 9 further comprises a collector C connected to a current mirror 12 via a line 13. The on chip transistor 9 comprises an emitter E connected to the internal node 6 and connected via the line 5 to the output 4 of the on chip current generator 3. The emitter E of the on chip transistor 9 is further connected via an on chip resistor 14 to a reference potential GND (Ground). The current mirror 12 is adapted to mirror the collector current I_C flowing through the collector C of the on chip transistor 9 to generate the temperature independent current I_out at the output terminal 2 of the on chip temperature independent current generator 1. The current mirror 12 is in a preferred embodiment a CMOS current mirror or a BJT current mirror. The on chip independent reference voltage generator 10 can be in a possible embodiment formed by an on chip reference voltage generator integrated on the chip. In an alternative embodiment, the reference voltage generator 10 can also be formed by an external voltage reference source. The current mirror 12 can be adapted to mirror, multiply and/or replicate the collector current I_C of the on chip bipolar NPN transistor 9.
• The on chip current generator 3 can be implemented in a possible exemplary embodiment by a circuit as illustrated in Fig. 2. In the illustrated embodiment, the on chip current generator 3 comprises an operation amplifier 15 having an inverting input (-) and a non-inverting input (+). In a possible embodiment, the inverting input (-) of the operation amplifier 15 is connected to a first bipolar transistor 16 and the non-inverting input (+) of the operation amplifier 15 is connected to a second bipolar transistor 17 via a resistor 18 as illustrated in Fig. 2. The resistor 18 comprises a predetermined resistance R_PTAT. The operation amplifier 15 comprises an output connected to an integrated CMOS current mirror 19 of the on chip current generator 3. The on chip resistor 14 as illustrated in Fig. 1 is in a preferred embodiment of the same type and/or material as the resistor 18 of the on chip current generator 3. The on chip resistor 14 comprises in a possible embodiment a resistance m*R_PTAT being m times the resistance R_PTAT of the resistor 18 of the on chip current generator 3 wherein m is an integer number equal or greater than 1. The resistance R_PTAT of the resistor 18 of the on chip current generator 3 as illustrated in Fig. 2 is in a possible embodiment dependent on the temperature T of the chip. In an alternative embodiment, the resistance R_PTAT of the resistor 18 of the on chip current generator 3 is temperature independent.
• The current I_out generated by the temperature independent current generator 1 is in a possible embodiment temperature independent in a wide temperature range between e.g. about -60° Celsius and about +200° Celsius. The generated temperature independent current I_out at the output terminal 2 of the on chip temperature independent current generator 1 can comprise in a possible embodiment a nominal current amplitude in a range of about 0,6 to 1,0 µAmp.
• As can be seen in the circuit diagram of Fig. 2, the base emitter voltage V _be1 of the bipolar transistor 16 is equal to the base emitter voltage V_be2 of the second bipolar transistor 17 reduced by the voltage drop across the resistor 18: $$V_{\mathit{be}1}=V_{\mathit{be}2}+V_{\mathit{rptat}}$$

  
• The current mirror 19 supplies the resistor 18 with a current s*I_PTAT as shown in Fig. 2 so that the voltage drop across the resistor 18 is given by: $$V_{\mathit{rptat}}=s*I_{\mathit{PTAT}}*R_{\mathit{PTAT}}$$

  
• The base emitter voltage V_be across the bipolar transistors 16, 17 is given as follows: $$V_{\mathit{be}1}={\phi }_T*1n\left(s*I_{\mathit{PTAT}}/I_s\right)$$

  

  $$V_{\mathit{be}2}={\phi }_T*1n\left(s*I_{\mathit{PTAT}}/\left(I_s*n\right)\right),$$

  

  wherein n is a ratio or a multiplication factor.
• Further, $${\phi }_T=\frac{K*T}{e},$$

  

  wherein
• K is a Boltzmann constant
• T is the temperature in Kelvin and
• e is the charge of an electron.
• I_s is the temperature current of a pn-junction of a bipolar transistor,
• s is the number of the current mirror sections in the PTAT current generator.
• Consequently: $$V_{\mathit{rptat}}=V_{\mathit{be}1}-V_{\mathit{be}2}$$

  

  $$s*I_{\mathit{PTAT}}*R_{\mathit{PTAT}}={\phi }_T\left(1n\left(s*I_{\mathit{PTAT}}/I_s\right)-1n\left(s*I_{\mathit{PTAT}}/\left(I_s*n\right)\right)\right)$$

  

  $$s*I_{\mathit{PTAT}}*R_{\mathit{PTAT}}={\phi }_T*1n\left(\frac{1}{n}\right)$$

  

  $$I_{\mathit{PTAT}}={\phi }_T*1n\left(n\right)/\left(s*R_{\mathit{PTAT}}\right)=\frac{\mathit{KT}}{e}\mathit{en}\left(n\right)/\left({\mathit{s\; R}}_{\mathit{PTAT}}\right)$$

  
• Expression (9) is a formula for calculating the generated current I_PTAT output by the on chip current generator 3 at the output 4 via the line 5 to the internal node 6 of the on chip temperature independent current generator 1. The generated electrical current I_PTAT depends on design parameters n, s, R_PTAT and a physical parameter, i.e. the temperature T in Kelvin.
• The resistor 14 is of the same type and/or material as the resistor 18 used for the PTAT current generator 3: $$R=m*R_{\mathit{PTAT}}$$

  

  wherein R is the resistance of resistor 14 and R_PTAT is the resistance of resistor 18 and m can be any positive real number.
• The output current I_out can be a replica or multiplied product of the current I_PTAT: $$I_{\mathit{OUT}}=l*I_{\mathit{PTAT}}$$

  

  wherein 1 is an integer number.
• The output current I_out has the same temperature dependency as the collector current I_C. Accordingly, it is sufficient to make the collector current I_C temperature independent.
• Based on the first Kirchhoff law and ignoring the base current I_B of the NPN transistor 9 gives: $$I_C+I_{\mathit{PTAT}}=I_R$$

  

  or $$I_C=I_R-I_{\mathit{PTAT}}$$

  

  with: $$I_R=\frac{V_R}{R}=\frac{V_R}{m*R_{\mathit{PTAT}}}$$

  

  and $$V_R=V_{\mathit{REF}}-V_{\mathit{BEQ}}$$

  
• The collector current I_C can be expressed as follows: $$I_C=\frac{V_{\mathit{REF}}-V_{\mathit{BEQ}}}{m*R_{\mathit{PTAT}}}-I_{\mathit{PTAT}}$$

  
• V_BEQ is the voltage between the base B and the emitter E terminals of the bipolar transistor 9. This voltage can have a negative temperature dependency ΔVbeq around 2 mV/Kelvin. Because of the small temperature dependency, it is possible to write: $$V_{\mathit{BEQ}}={\mathrm{<figure-callout\; id="0"\; label="V\; BEQ"\; filenames="imgf0003.png"\; state="\{\{state\}\}">V</figure-callout>}}_{\mathit{BEQ}}-T*{\mathrm{\Delta }\mathit{V}}_{\mathit{BEQ}},$$

  

  wherein V_BEQ0 is the emitter-base voltage of the transistor 9 at 0°K.
• Using the equation (9) one can re-write equation (16) in the following way: $$I_C=\frac{V_{\mathit{REF}}-V_{\mathit{BEQ}0}}{m*R_{\mathit{PTAT}}}+\frac{T*{\mathrm{\Delta }\mathit{V}}_{\mathit{BEQ}}}{m*R_{\mathit{PTAT}}}-\frac{K*T}{e}*1n\left(n\right)*/\left(s*R_{\mathit{PTAT}}\right)$$

  
• The resistance of the resistor 18 can be either temperature independent or temperature dependent.
• To provide a temperature independent current by the on chip temperature independent current generator 1 it is necessary that the collector current I_C is temperature independent. By differentiating both sides of equation 18 with the temperature T and by assuming that the reference voltage V_REF provided by the temperature independent reference voltage generator 10 and the voltage V_BEQ0 are constant one arrives to the following equation: $$0=\frac{{\mathrm{\Delta }\mathit{V}}_{\mathit{BEQ}}}{m*R_{\mathit{PTAT}}}-\frac{K}{e}*1n\left(n\right)/\left(s*R_{\mathit{PTAT}}\right)$$

  
• Equation (19) can be rewritten as: $$\frac{{\mathrm{\Delta }\mathit{V}}_{\mathit{BEQ}}}{m}=\frac{K}{e*s}*1n\left(n\right)$$

  
• Equation (20) can be rewritten as follows: $$\frac{s}{m*1n\left(n\right)}=\frac{K}{e*{\mathrm{\Delta }\mathit{V}}_{\mathit{BEQ}}}$$

  
• Accordingly, by knowing the voltage ΔVbeq from a technology specification and by fixing two of the three free selectable design parameters m, n, s, it is possible to determine the third design parameter from equation (21) such that the collector current I_C is temperature independent.
• In a second alternative embodiment, the resistance R_PTAT of the resistor 18 is temperature dependent. In this case, the resistance of the resistor can have a first order temperature coefficient T_C. Further, order temperature coefficients can be ignored because of their small influence.
• The resistance R_PTAT of the resistor 18 can be written as follows: $$R_{\mathit{PTAT}}={\mathrm{<figure-callout\; id="0"\; label="R\; PTAT"\; filenames="imgf0003.png"\; state="\{\{state\}\}">R</figure-callout>}}_{\mathit{PTAT}}*\left(1+T_C*T\right)$$

  

  wherein R_PTAT0 is the resistor value at 0°K.
• Rewriting equation (18) leads to the following equation: $$I_C=\frac{V_{\mathit{REF}}-V_{\mathit{BEQ}0}}{m*{\mathrm{<figure-callout\; id="0"\; label="R\; PTAT"\; filenames="imgf0003.png"\; state="\{\{state\}\}">R</figure-callout>}}_{\mathit{PTAT}}*\left(1+T_C*T\right)}+\frac{T*{\mathrm{\Delta }\mathit{V}}_{\mathit{BEQ}}}{m*{\mathrm{<figure-callout\; id="0"\; label="R\; PTAT"\; filenames="imgf0003.png"\; state="\{\{state\}\}">R</figure-callout>}}_{\mathit{PTAT}}*\left(1+T_C*T\right)}-\frac{K*T}{e*s*{\mathrm{<figure-callout\; id="0"\; label="R\; PTAT"\; filenames="imgf0003.png"\; state="\{\{state\}\}">R</figure-callout>}}_{\mathit{PTAT}}*\left(1+T_C*T\right)}*1n\left(n\right)$$

  

  which can be rewritten into: $$I_C=\frac{V_{\mathit{REF}}-V_{\mathit{BEQ}0}}{m*{\mathrm{<figure-callout\; id="0"\; label="R\; PTAT"\; filenames="imgf0003.png"\; state="\{\{state\}\}">R</figure-callout>}}_{\mathit{PTAT}}*\left(1+T_C*T\right)}+\frac{T*{\mathrm{\Delta }\mathit{V}}_{\mathit{BEQ}}}{m*{\mathrm{<figure-callout\; id="0"\; label="R\; PTAT"\; filenames="imgf0003.png"\; state="\{\{state\}\}">R</figure-callout>}}_{\mathit{PTAT}}*\left(1+T_C*T\right)}-\frac{K*T*m}{e*s*m*{\mathrm{<figure-callout\; id="0"\; label="R\; PTAT"\; filenames="imgf0003.png"\; state="\{\{state\}\}">R</figure-callout>}}_{\mathit{PTAT}}*\left(1+T_C*T\right)}*1n\left(n\right)$$

  
• Since m*R_PTAT0 is constant, both sides of equation (24) can be multiplied with this value: $$I_C*m*{\mathrm{<figure-callout\; id="0"\; label="R\; PTAT"\; filenames="imgf0003.png"\; state="\{\{state\}\}">R</figure-callout>}}_{\mathit{PTAT}}=\frac{V_{\mathit{REF}}-{\mathrm{<figure-callout\; id="0"\; label="V\; BEQ"\; filenames="imgf0003.png"\; state="\{\{state\}\}">V</figure-callout>}}_{\mathit{BEQ}}}{\left(1+T_C*T\right)}+\frac{T*{\mathrm{\Delta }\mathit{V}}_{\mathit{BEQ}}}{\left(1+T_C*T\right)}-\frac{K*T*m}{e*s*\left(1+T_C*T\right)}*1n\left(n\right)$$

  
• Differentiating equation (25) on both sides with the temperature T gives: $$0=\frac{V_{\mathit{REF}}-{\mathrm{<figure-callout\; id="0"\; label="V\; BEQ"\; filenames="imgf0003.png"\; state="\{\{state\}\}">V</figure-callout>}}_{\mathit{BEQ}}*T_C}{{\left(1+T_C*\mathrm{<figure-callout\; id="2"\; label="T"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">T</figure-callout>}\right)}^2}+\frac{{\mathrm{\Delta }\mathit{V}}_{\mathit{BEQ}}}{{\left(1+T_C*\mathrm{<figure-callout\; id="2"\; label="T"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">T</figure-callout>}\right)}^2}-\frac{K*m*1n\left(n\right)}{e*s*{\left(1+T_C*\mathrm{<figure-callout\; id="2"\; label="T"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">\; <figure-callout\; id="0"\; label="T"\; filenames="imgf0003.png"\; state="\{\{state\}\}">T</figure-callout>\; </figure-callout>}\right)}^2}$$

  

  and $$0=\frac{e*s*{\mathrm{\Delta }\mathit{V}}_{\mathit{BEQ}}-T_C*\left(V_{\mathit{REF}}-V_{\mathit{BEQ}0}\right))-K*m*1n\left(n\right)}{e*s*{\left(1+T_C*\mathrm{<figure-callout\; id="2"\; label="T"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">T</figure-callout>}\right)}^2}$$

  

  or $$e*s*\left({\mathrm{\Delta }\mathit{V}}_{\mathit{BEQ}}-T_C\left(V_{\mathit{REF}}-{\mathrm{<figure-callout\; id="0"\; label="V\; BEQ"\; filenames="imgf0003.png"\; state="\{\{state\}\}">V</figure-callout>}}_{\mathit{BEQ}}\right)\right)-K*m*1n\left(n\right)=0$$

  

  $$e*s*\left({\mathrm{\Delta }\mathit{V}}_{\mathit{BEQ}}-T_C\left(V_{\mathit{REF}}-{\mathrm{<figure-callout\; id="0"\; label="V\; BEQ"\; filenames="imgf0003.png"\; state="\{\{state\}\}">V</figure-callout>}}_{\mathit{BEQ}}\right)\right)-K*m*1n\left(n\right)$$

  
• From this follows: $$\frac{s}{m*1n\left(n\right)}=\frac{K}{e*\left({\mathrm{\Delta }\mathit{V}}_{\mathit{BEQ}}-T_C\left(V_{\mathit{REF}}-{\mathrm{<figure-callout\; id="0"\; label="V\; BEQ"\; filenames="imgf0003.png"\; state="\{\{state\}\}">V</figure-callout>}}_{\mathit{BEQ}}\right)\right)}$$

  
• Consequently, by knowing ΔVbeq, V_BEQ0 and the temperature coefficient T_C from the technology specification, it is possible by fixing two of the three free selectable design parameters m, n, s to determine the third design parameter from equation (29) such that the collector current I_C becomes temperature independent.
• Fig. 3 is a diagram illustrating the temperature dependency of an electrical current generated by a conventional current generator and by an embodiment of an on chip temperature independent current generator 1 according to the present invention. The curve I illustrates the current I generated by the on chip temperature independent current generator 1 in a wide temperature range between about -60° Celsius and about +200° Celsius. Curve II illustrates an electrical current provided by a conventional current generator. As can be seen from the curves illustrated in Fig. 3, the current I_out generated by the temperature independent current generator 1 according to the present invention (curve I) is almost completely temperature independent in the wide temperature range between -60° Celsius and +200° Celsius. In contrast, the conventional current generator (curve II) generates a temperature dependent current. With increasing temperature, the current generated by the conventional current generator increases steadily. Fig. 3 shows a simulation plot of the generated currents depending on temperature T of the chip. As can be seen from Fig. 3, the current I_out generated by the temperature independent current generator 1 comprises a nominal current amplitude of about 0,8 µAmp, i.e. in a range of about 0,6 to 1,0 µAmp.

Claims (9)

1. An on chip temperature independent current generator, TICG, (1) for generating a temperature independent current (I_out),
   said TICG (1) comprising:

   - an on chip current generator (3) having an output (4) to provide an electrical current (I_PTAT) being proportional to an absolute temperature (T) of said chip;

   - an on chip transistor (9) having a base (B) connected to a temperature independent reference voltage generator (10), a collector (C) connected to a current mirror (12) and an emitter (E) connected to the output (4) of the on chip current generator (3) and connected via an on chip resistor (14) to a reference potential (GND);

   - wherein the current mirror (12) is adapted to mirror a collector current (I_c) flowing to the collector (C) of said on chip transistor (9) to generate the temperature independent current (I_out).
2. The on chip temperature independent current generator according to claim 1, wherein said on chip transistor (9) is an on chip bipolar NPN transistor.
3. The on chip temperature independent current generator according to claim 1 and 2, wherein said current mirror (12) is a CMOS or BJT current mirror.
4. The on chip temperature independent current generator according to any of the preceding claims 1 to 3, wherein said on chip current generator (3) comprises an operation amplifier (15) having an inverting input (-) to which a first bipolar transistor (10) is connected and a non-inverting input (+) to which a second bipolar transistor (17) is connected via a resistor (18) having a predetermined resistance (R_PTAT) and having an output connected to an integrated CMOS current mirror (19) of said on chip current generator (3).
5. The on chip temperature independent current generator according to claim 4, wherein said on chip resistor (14) is of the same type as the resistor (18) of said on chip current generator (3).
6. The on chip temperature independent current generator according to claim 5, wherein said on chip resistor (14) has a resistance (mR_PTAT) being m times the resistance (R_PTAT) of the resistor (18) of said on chip current generator (3), wherein m is a positive real number.
7. The on chip temperature independent current generator according to any of the preceding claims 1 to 6, wherein the resistance (R_PTAT) of the resistor (18) of said on chip current generator (3) is dependent on the temperature (T) of said chip.
8. The on chip temperature independent current generator according to any of the preceding claims 1 to 6, wherein the resistance (R_PTAT) of the resistor (18) of said on chip current generator (3) is temperature independent.
9. The on chip temperature independent current generator according to any of the preceding claims 1 to 8, wherein the current (I_out) generated by said temperature independent current generator, TICG, (1) is temperature independent in a wide temperature range between about -60° Celsius and about +200° Celsius.

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